Field of the Invention
The present invention relates to products, systems, and methods for controlling leaf cutter ants (LCA), significant pests of agriculture and forestry throughout the southern U.S., Central and South America. More specifically, this invention relates to methods and systems for 1) inducing foraging leaf cutter ants to orient to, collect, and transport back to their nests a fungicide-laced bait material, 2) inducing these ants to provide the material to the fungal gardens upon which the colony relies as its sole food source, and 3) producing mortality in or halting growth of the fungal colony, thereby depriving the ants of their only sustenance and causing all individuals therein to starve to death. All aspects of the invention will contribute to elimination of LCA colonies within the treated area, consequently reducing the damage caused by these insects to susceptible agricultural crops and forested land.
Background of the Invention
Leaf-cutting ants (LCA), a group which comprises approximately 40 different species of ants, range from the southern U.S. (Texas, Louisiana, and New Mexico) through Central America and in South America, as far south as Argentina, and are considered some of the most impactful herbivores of their ecosystem. In tropical forests, it is estimated that LCA of the genus Atta consume 12 to 17% of all leaves produced in the ecosystem, while grass-cutting ants have been found to significantly impact pasture growth, reducing this land's commercial value by up to 10% in Paraguay and Brazil. A single LCA nest may contain as many as 2 million individual insects, occupy 1 acre in space and extend up to 25 feet below ground, and displace 40,000 kg of soil during excavation. One study estimated that a single species (Atta cephalotes L.) was capable of executing a complete turnover of all soil in a Costa Rican rainforest over a period of 2 to 3 centuries. Through these activities, LCA have a profound effect on their environment, impacting microclimate, forest structure and regeneration.
The most northern-distributed member of the Atta genus, the Texas leaf-cutting ant (Atta texana Buckley), is currently found in 129 counties in the state of Texas and 13 parishes in Louisiana, and causes a significant degree of economic damage throughout this range. In Texas, LCA are pests of multiple fruit and nut crops, including peach, plum, blackberry, and citrus; cereal crops; and ornamental plants, as well as many key forestry species. Certain species of Atta are so voracious in their leaf-cutting activity that a single colony can completely defoliate entire citrus trees in a period of less than a day. Agricultural losses across Louisiana and Texas due to LCA activity typically hover around $5 million each year. While LCA usually prefer broad-leaved trees, they will target pines during the winter, when their preferred host plants are unavailable, and can cause significant damage to young pines when ant populations are high, particularly in recent plantings. This makes them a major pest of concern to reforestation programs. At heavily infested sites, LCA can damage pine seedlings to the point of death in only a few days, rendering natural tree reproduction impossible unless the ant population can be adequately controlled. The Texas leaf-cutter ant kills pine seedlings equivalent to an average area of 12,000 acres each year, resulting in an average of $2.3 million in costs for replanting and ant control. While creating their massive nests, these insects can also physically damage the stability of roads and farmland. Throughout LCA's range across North and South America—where the ants are pests of multiple commercial crops, such as coffee, banana, mango, cacao, citrus, and others—these insects are responsible for an approximate total of $1 billion in yearly losses.
Leaf cutter ants can travel 600 feet or farther from their nest in search of appropriate plant material, forming foraging trails along which the insects travel back and forth. Once a suitable plant is located, the ants attack en masse, sometimes in numbers large enough to strip an entire small or medium-sized tree in a single night. The ants cut fragments of leaves and carry them in their mandibles back to the entrance of the nest. The ants do not actually feed upon these leaves, but cut them into small pieces to provide a suitable food source for a symbiotic fungus which they cultivate within special chambers in the nest. Leaf cutter ants feed on certain parts of this fungus as their sole food source, both as larvae and adult insects. In order to tend this fungal garden, the ants not only provide plant material for nutrition, they also produce enzymes and amino acids to support the growth of the fungus, as well as substances that prevent the growth of competing fungi within the nest. The ants and the fungus are entirely dependent upon one another for survival. The ants provide a unique habitat in which the fungus can thrive, providing and maintaining an environment that is free of all potential competition, as well as any microorganism or pathogen that could do it harm, while the ants rely on the fungus as their only food source. Hence, the absence of one would soon result in the disappearance of the other.
Control of LCA has proved challenging, due to the protection provided to the colony by their massive and complex underground nests. The organobromine insecticide methyl bromide was successfully used against LCA for over half a century, but the U.S. Environmental Protection Agency (EPA) discontinued its use against these ants in 2005 (most other uses have also been phased out) due to its potential risk of ozone depletion. Plants susceptible to attack by LCA can be protected for short periods through application of contact insecticides, including carbaryl or permethrin, but these treatments must be repeated frequently in order to maintain this protective effect, and make no impact on the survival of LCA colonies in the area. Furthermore, because ants feed only on the fungus cultivated within their nests, conventional ant baits typically do not work as effectively for LCA as for other species of ant pests, such as fire ants. The only ant bait currently available for LCA control is a product called Amdro® Ant Block, a formulation of the insecticide, hydramethylnon, which was originally designed for fire ants. This product has been identified as relatively safe, and can be applied to lawns, ornamental gardens, roadsides, etc. However, this product has several shortcomings. It has a relatively brief shelf life, and so cannot be stored for long periods; and while it does suppress LCA activity, it does not consistently eradicate the nest. In roughly half of cases, LCA activity rebounds 4 to 6 months after application, though typically not to pre-treatment levels. Perhaps most limiting, this product cannot be applied on agricultural land. The same is true for an injection method of fipronil, which has been approved for forestry management of LCA, but is currently not permitted for other applications.
There is an urgent need to develop new ways to control these ants, to address the economic threat they pose, but also the preservation of the environment, to reduce residual effects caused by the indiscriminate use of chemicals. A highly promising alternative to conventional chemical insecticide-based management is the use of natural products from plants potentially toxic to the fungus and/or the ants. Attempts have been made to create bait formulations using organic plant materials (dehydrated citrus pulp, corn, eucalyptus leaves, cassava flour, wheat flour, soy bran, molasses) coupled with sugar-based attractants (glucose, fructose, or sucrose), with the purpose of causing mortality of the specific caste of ants responsible for the maintenance and rearing of the symbiotic fungus. Because the ants are so entirely dependent on their fungal colonies for sustenance, and because the responsibility for their care is placed only on a particular caste of individuals within the nest, the elimination of this caste is intended to trigger a deadly chain of events, eventually resulting in the annihilation of the colony. As the ants responsible for the maintenance of the colony begin to die off, the health of the symbiotic fungus begins to deteriorate, since there are no other individuals within the colony to replace the care workers who have been lost. Eventually, this deterioration progresses to such a degree that the fungus becomes unsuitable for consumption by the ants, and all members of the colony, including the queen, starve to death.
The present invention aims to achieve this end through a more direct approach to LCA control, targeting the fungus itself, rather than the ants responsible for caring for it. This invention consists of a bait pellet formulation designed to attract LCAs and induce them to pick up the pellet and carry it back to their nests. However, in place of an insecticidal agent intended to produce mortality in the ants, our bait formulation will be impregnated with a blend of natural botanical fungicides (wintergreen oil, methyl salicylate, and their constituents, concentrations ranging from 0.1 to 75%) that are highly attractive to the ants, exerting no repellent effects—a critical advantage over many insecticides, as any level of repellency could prevent introduction of these fungicides into the nest. Once these pellets are carried into the nest, directly to the chambers in which LCA fungal gardens are cultivated, the incorporated fungicidal and fungistatic agents will infect the fungal garden, halting its growth and eventually resulting in its death. Without the fungus to rely upon as a food source, the ant colony will quickly follow.
Results of testing with these fungicidal and fungistatic materials have demonstrated that exposure to wintergreen oil produced total annihilation of the fungus, and both wintergreen oil and methyl salicylate can suppress 100% of fungal growth under laboratory conditions. Ongoing field tests of the invention have also produced encouraging results: bait pellets impregnated with wintergreen oil suppressed activity of LCA colonies in Brazil to zero within an interval of 42 to 63 days after application. In addition to this excellent degree of efficacy against the target insects, this invention presents multiple advantages over current LCA control strategies. This bait formulation does not contain any amount of conventional chemical insecticide, relying instead on a blend of botanical oils, which, in spite of their fungicidal activity, have no negative impacts on people, on non-target species (with the exception of other fungi), or on the environment. Wintergreen oil has actually been used as an herbal pain remedy throughout North American history. Application of this invention will help to reduce the impact of LCA cutting and nest building activity, protecting the value of susceptible agricultural crops and forestry species throughout its geographic range, without resorting to additional inputs of potentially harmful conventional pesticides.